The inventin relates to a process for the separation of straight paraffins from a mixture mainly consisting of straight and branched paraffins (the feed) by selective adsorption of the straight paraffins on a molecular sieve, followed by desorption of the straight paraffins from the molecular sieve with the aid of hydrogen.
For use in gasolines, unbranched saturated non-cyclic aliphatic hydrocarbons (hereinafter called straight paraffins) are less suitable, because they have a lower octane number than branched saturated non-cyclic aliphatic hydrocarbons (hereinafter called branched paraffins) having the same number of carbon atoms. In order to prepare gasolines having a high octane number, methods have therefore been developed for the separation of the straight paraffins from a mixture containing straight and branched paraffins.
A known method to this end consists in passing a mixture mainly consisting of straight and branched paraffins (the feed) through a molecular sieve bed, which sieve adsorbs the straight but not the branched paraffins. When the molecular sieve is sufficiently loaded with straight paraffins (this will generally be shortly before the point at which not all straight paraffins would be adsorbed from the feed) the supply of feed is interrupted and hydrogen is passed through the molecular sieve bed in order to desorb the straight paraffins.
In the operation of this process a number of difficulties arise. When the supply of feed is replaced by counter-current hydrogen supply for the desorption of the straight paraffins the feed still present in the voids in the bed will be expelled by the hydrogen, and leave the bed together with the desorbed straight paraffins. This of course leads to a loss of capacity of the whole plant, because part of the supplied feed leaves the bed again in unpurified state. For that reason, after the desired quantity of straight paraffins has been absorbed, a co-purge step is employed, in which the supply of feed is replaced by supply of a mixture of hydrogen and straight paraffins, which mixture is passed through the molecular sieve bed in the same direction as the feed. The straight paraffins still present in the feed are adsorbed in the last part of the bed, and branched paraffins leave the bed. When all the branched paraffins have been expelled, the bed is fully filled with a mixture of hydrogen and straight paraffins, which latter may be adsorbed or non-adsorbed. Desorption of straight paraffins can now be effected by passing hydrogen through the bed counter-currently to the direction of the co-purge (and therefore of the feed).
Here again, problems are encountered which differ according to whether the feed is passed through the bed downwardly or upwardly.
If the feed is passed downwardly through the bed, the latter is charged with hydrogen after the preceding regeneration. Because the feed has a considerably higher specific gravity than hydrogen, channeling occurs during the passage through the bed, as a result of which the hydrogen is not expelled to a sufficient degree from the voids in the bed. Consequently optimum contact of the feed with the surface of the molecular sieve in the bed does not take place, as a result of which not all sites available on the molecular sieve for the adsorption of straight paraffin adsorption are utilized and the total amount of straight paraffins adsorbed remains far below the possible and desired level.
If the feed is passed upwardly through the bed and the co-purge (which consists of a mixture of hydrogen and straight paraffins) as well, the co-purge gas, which has a lower specific gravity than the feed (and also than the branched paraffins therein) finds the shortest way to the top, as a result of which channeling takes place and the feed or the branched paraffins therefrom are not expelled from the bed in an adequate manner. Another known method is described in British Pat. No. 1,067,171 which employs pressure reduction rather than introduction of a gas stream for co-purge and desorption.
The invention provides a process in which the afore-mentioned problems do not occur.